Large substrates with electronically active components distributed over the extent of the substrate are useful in a variety of electronic systems, for example flat-panel imaging devices such as flat-panel liquid crystal or OLED display devices and in digital radiographic plates. Large substrates with electrically active components are also found in flat-panel solar cells.
The electronically active components are typically formed by sputtering a layer of inorganic semiconductor material or by spin-coating organic material over the entire substrate. Inorganic semiconductor materials can be processed to improve their electronic characteristics, for example amorphous silicon can be treated to form low-temperature or high-temperature poly-crystalline silicon. In other process methods, microcrystalline semiconductor layers can be formed by using an underlying seeding layer. These methods typically improve the electron mobility of the semiconductor. The substrate and layer of semiconductor material are typically photo-lithographically processed to define electronically active components, such as transistors. Such transistors are known as thin-film transistors (TFTs) since they are formed in a thin layer of semiconductor material, typically silicon. In these devices, the substrate is often made of glass, for example Corning Eagle or Jade glass designed for display applications. Photo-lithographic methods used to form the active components are known in the art.
These traditional techniques have significant limitations. Despite processing methods used to improve the performance of thin-film transistors, such transistors have performance that is lower than the performance of traditional integrated circuits formed in mono-crystalline semiconductor material. Also, semiconductor material and active components can be needed only on portions of the substrate, leading to wasted material and increased material and processing costs. The substrate materials can be limited by the processing steps necessary to process the semiconductor material and the photo-lithographic steps used to pattern the active components. For example, plastic substrates have a limited chemical and heat tolerance and do not readily survive photo-lithographic processing. Furthermore, the manufacturing equipment needed to process large substrates with thin-film circuitry is relatively expensive.
In an alternative manufacturing technique, a mono-crystalline semiconductor wafer is employed as the substrate. While this approach can provide substrates with similar performance as integrated circuits, the size of such substrates may be limited, for example, to a 12-inch diameter circle, and the wafers are relatively expensive compared to other substrate materials such as glass or polymer.
In yet another approach, thin layers of semiconductor are bonded to a substrate and then processed. Such a method is known as semiconductor-on-glass or silicon-on-glass (SOG) and is described, for example, in U.S. Patent Application 20080224254, dated Sep. 18, 2004. If the semiconductor material is crystalline, high-performance thin-film circuits can be obtained. However, the bonding technique can be expensive and the processing equipment for the substrates to form the thin-film active components on large substrates remains relatively expensive.
Publication number 11-142878 of the Patent Abstracts of Japan entitled “Formation of Display Transistor Array Panel” describes etching a substrate to remove it from a thin-film transistor array on which the TFT array was formed. TFT circuits formed on a first substrate can be transferred to a second substrate by adhering the first substrate and the TFTs to the surface of the second substrate and then etching away the first substrate, leaving the TFTs bonded to the second substrate. This method requires etching a significant quantity of material and risks damaging the exposed TFT array.
An alternative method of locating material on a substrate is described in U.S. Pat. No. 7,127,810. In this method, a first substrate carries a thin-film object to be transferred to a second substrate. An adhesive is applied to the object to be transferred or to the second substrate in the desired location of the object. The substrates are aligned and brought into contact. A laser beam irradiates the object to abrade the transferring thin film so that the transferring thin film adheres to the second substrate. The first and second substrates are separated, peeling the film in the abraded areas from the first substrate and transferring it to the second substrate. In one embodiment, a plurality of objects are selectively transferred by employing a plurality of laser beams to abrade selected area. Objects to be transferred can include thin-film circuits. In addition, U.S. Pat. No. 6,969,624 describes a method of transferring a device from a first substrate onto a holding substrate by selectively irradiating an interface with an energy beam. The interface is located between a device for transfer and the first substrate and includes a material that generates ablation upon irradiation, thereby releasing the device from the substrate. For example, a light-emitting device (LED) may be made of a nitride semiconductor on a sapphire substrate. The energy beam is directed to the interface between the sapphire substrate and the LED nitride semiconductor releasing the LED and allowing the LED to adhere to a holding substrate coated with an adhesive. The adhesive is then cured. These methods, however, typically require the patterned deposition of adhesive on the object(s) or on the second substrate. Moreover, the laser beam that irradiates the object should be shaped to match the shape of the object, and the laser abrasion can damage the object to be transferred. Furthermore, the adhesive cure takes time, which typically reduces the throughput of the manufacturing system.
In another method for transferring active components from one substrate to another, described in “AMOLED Displays using Transfer-Printed Integrated Circuits” published in the Proceedings of the 2009 Society for Information Display International Symposium Jun. 2-5, 2009, in San Antonio Tex., US, vol. 40, Book 2, ISSN 0009-0966X, paper 63.2 p. 947, small integrated circuits are formed in a wafer and released from the wafer by etching beneath the circuits. A PDMS stamp is pressed against the wafer and the circuits adhered to the stamp. The circuits are then pressed against a substrate coated with an adhesive, adhered to the substrate, and the adhesive is subsequently cured. This method, however, is susceptible to particulate contamination that can affect the adhesion of the integrated circuits to the stamps, the adhesion of the integrated circuits to the coated substrate, and the position of the integrated circuits on the substrate.